The present invention relates to a sensor control apparatus and method for controlling an air/fuel ratio sensor that detects a concentration of a particular gas component of an exhaust gas emitted from an internal combustion engine and particularly to a sensor control apparatus and method capable of attaining earlier or accelerated start of an air/fuel ratio control after start of the engine.
For an air/fuel ratio control of an internal combustion engine, a sensor control apparatus having an air/fuel ratio sensor for detecting an oxygen concentration in an exhaust gas emitted from an internal combustion engine (air/fuel ratio) has been put to practical use. Generally known as such an air/fuel ratio sensor are a sensor (so-called λ sensor) that produces a binary output in response to an oxygen concentration in an exhaust gas (i.e., in response to a rich or lean air/fuel ratio) and a sensor (pump current type or limiting current-type oxygen sensor) that produces a nearly linear output over a wide range of oxygen concentration. In recent years, to comply with stringent exhaust gas regulations of an internal combustion engine, it is required to control the air/fuel ratio accurately and therefore a pump current type or limiting current-type oxygen sensor have been used in place of the λ sensor.
In either of the sensors is used, as a principle, a sensor cell having a pair of electrodes on the opposite sides of a solid electrolyte (e.g., ZrO2) so as to utilize such a phenomenon that an electromotive force is produced when the ambient gases on the opposite sides of the sensor cell are different in oxygen concentration and oxygen ion moves between the opposite sides through the solid electrolyte when current is supplied between the electrodes. Such a phenomenon is not attained unless the solid electrolyte is heated and activated. In the meantime, there may occur such a case where it takes a long time from over ten seconds to tens of seconds for the pump current type or the limiting current-type oxygen sensor that can detect an oxygen concentration over a wide range, to become activated.
Generally, a sensor control apparatus is configured to be able to detect whether the sensor has reached an activated condition. The technique for detecting such an activated condition is disclosed in Japanese Unexamined Patent Publication Nos. 4-313056, 10-104195 and 9-170997.